There are a variety of processes which utilize alkali-based chemicals such as sodium hydroxide in the pulping, bleaching or oxidation of wood materials. These processes include chemical and semi-chemical methods for breaking down wood chips or other wood-based starting materials into wood fiber for the production of paper, cardboard and similar cellulose-based products. Other processes which use such alkali-based chemicals include the oxidizing and bleaching of wood pulp for paper production.
In a typical chemical-based wood pulping process, such as the kraft process, wood chips are treated with an aqueous solution of mainly sodium hydroxide (caustic soda) to separate out lignin and other organic constituents which bind the cellulose fibers together in order that the wood can be broken down into individual fibers for various uses such as paper making. In the kraft process this solution also contains sodium sulfide. The sodium hydroxide reacts and combines chemically with lignin forming an organic-based solution referred to as black liquor or spent liquor. The black liquor is separated from the fiber and burned in a recovery boiler to recover heat from the organics. In the process of burning, the black liquor is converted into smelt, a molten phase in which the sodium-organic complex has been converted to sodium carbonate. In the kraft process sodium sulfide is also formed. In order to regenerate sodium for reuse in the pulping process, sodium carbonate must be converted back to sodium hydroxide or xe2x80x9crecausticizedxe2x80x9d. The molten smelt is typically dispersed with steam as it is poured into an aqueous solution, such as recycled dilute white liquor, weak wash or water, in which it dissolves to form a sodium carbonate solution referred to as green liquor due to the dark green appearance caused by the presence of an insoluble residue known as dregs. In some operations, such as in the soda process, the smelt is cooled and solidified prior to dissolution. The green liquor is sent to a causticizer where sodium carbonate is converted back to sodium hydroxide, thus producing xe2x80x9cwhite liquorxe2x80x9d for reuse in the wood pulping process. This process, known as causticization, is accomplished by the reaction of sodium carbonate with calcium hydroxide, also known as hydrated or slaked lime, in the green liquor. In the process of regenerating sodium hydroxide, the calcium hydroxide is converted to calcium carbonate, as a precipitate (also known as lime mud), which is then converted back to calcium hydroxide in a separate lime recovery circuit, also known as a lime recovery cycle, so that it can be reused in the causticizer again. In the lime recovery circuit calcium carbonate is burned in a kiln to drive off carbon dioxide as a gas, converting the calcium carbonate to calcium oxide, which is then hydrated with water in the green liquor to reform calcium hydroxide which can be reused in the causticization step.
An alternative method of recausticization which does not require the use of lime and the associated lime recovery process was developed in the 1970""s by Jan Janson, a researcher in Finland (U.S. Pat. No. 4,116,759). Janson proposed that sodium carbonate in the smelt could be causticized automatically (xe2x80x9cautocausticizedxe2x80x9d) in the recovery boiler by the addition of borate to the wood pulping circuit, thus eliminating the need for subsequent recausticization by calcium hydroxide and the accompanying lime recovery circuit.
The chemical reactions proposed by Janson for the autocausticization process were:
(1) Cooking or bleaching (delignification):
Na2HBO3LignOH⇄LignONa+NaH2BO3 
(2) Combustion:
2LignONa+x.O2xe2x86x92Na2CO3+y.CO2+zH2O 
(3) Autocausticization:
2 NaH2BO3+Na2CO3xe2x86x922 Na2HBO3+CO2+H2O 
In autocausticizing, sodium metaborate acts like a catalyst, in that it will react with sodium carbonate in the smelt to produce a more basic disodium borate and carbon dioxide. When the disodium borate is dissolved in water, it is hydrolyzed to regenerate sodium hydroxide and the original sodium metaborate, hence eliminating the need for lime and the lime kiln and associated lime recovery cycle.
In a typical kraft process, sodium hydroxide is recovered for reuse in the process using the traditional lime recausticization methods described above. Autocausticization offers several potential benefits over recausticization with lime. These include elimination of the capital costs associated with the lime recovery circuit, reduction of energy costs by elimination of the need to burn the calcium carbonate to release carbon dioxide and elimination of other operating costs associated with the lime recovery circuit. Alternatively, in some operations, where sodium is not being recovered and reused in the process, autocausticization offers significant potential cost savings due to reduced chemical requirements, since borate is not used up in the process, but is instead returned to the start of the process for reuse along with the regenerated sodium hydroxide. However, Janson teaches in the ""759 patent that it is essential to keep the sodium to boron molar ratio equal to or less than 2 (Na/Bxe2x89xa62) in order to ensure complete causticization.
Large scale trials (Janson, Jan and Bengt Arhippainen, xe2x80x9cMill Scale Development of the Borate-Based Kraft Pulping Processxe2x80x9d, International Conference on Recovery of Pulping Chemicals, Vancouver, British Columbia, Canada, Sep. 22-25, 1981) were conducted in the early 1980""s to investigate the commercial applicability of autocausticization using borate. However, operating difficulties were encountered and the process was never adopted on a commercial basis. Such difficulties are largely related to changes in the physical properties of the black liquor due to the presence of high levels of borate, such as large increases in the dissolved solids content and viscosity, leading to difficulties with spraying and droplet size in the recovery boiler, reduced evaporation rate and the transporting of the liquor from the digestor to the recovery boiler. Also, a reduction in the heating value of the black liquor may require the addition of supplemental fuel in the recovery boiler.
Despite the potential benefits offered by autocausticization, it has not been adopted commercially in view of the problems associated with the process. It is an object of this invention to provide an improved causticization process which will provide some of the significant benefits of autocausticization, while minimizing the difficulties associated with it.
This invention provides an improved method for causticizing sodium carbonate-containing smelt resulting from the combustion of black liquors, wherein a limited amount of borate is added such that only a portion of the sodium carbonate is autocausticized. The method of this invention provides reduced borate deadload in the circuit, resulting in improved recovery boiler operating conditions such as reduced black liquor viscosities and higher reaction efficiencies compared with full autocausticization at higher ratios of sodium to boron. This invention further provides a method for recausticization of sodium carbonate-containing smelts, wherein partial autocausticization is used in combination with lime recausticization to achieve improved conversion of sodium carbonate back to sodium hydroxide upon hydration of the smelt and lime recausticizing.
According to this invention an improved process is provided for causticizing sodium carbonate-containing smelt at sodium to boron molar ratios exceeding 3:1, wherein the amount of borate used is less than the stoichiometric requirement for complete autocausticization of all of the alkali carbonate present in the black liquor. It has been found that the autocausticization reaction can proceed with unexpectedly high efficiencies under these conditions. Further, it has been observed that this process of partial autocausticization occurs at a rate which exceeds 100% stoichiometric efficiency at low levels of borate addition, based on the autocausticization reactions proposed in the ""579 patent.
According to Janson""s proposed reactions two moles of boron are consumed per mole of sodium carbonate recovered, as shown in equation (3) above. At boron addition levels equivalent to about 52% of the stoichiometric requirements for full autocausticization of sodium carbonate, and at a sodium to boron molar ratio of about 2.9:1, the observed reaction efficiencies averaged about 86%. However, in tests at low levels of borate addition, equivalent to 5% and 10% of full autocausticization requirements, the conversion of sodium carbonate to sodium hydroxide was determined to be 9-17% and 15-17%, respectively, which is significantly above the theoretical 100% reaction efficiency. The sodium to boron molar ratios in these tests were about 20:1 and 11:1, respectively. This suggests that under these conditions of low borate addition and high sodium to boron molar ratios the autocausticization reaction may lead to the formation of a different borate composition than was proposed in the ""579 patent. In particular, the reaction product is believed to be Na3BO3 (trisodium borate), rather than Na2HBO3 (disodium borate, also written as Na4B2O5) which was proposed by Janson. As a result, a higher level of autocausticization is achieved for a given level of borate used.
Partial autocausticization may occur to some extent in both the gas phase and the smelt. The reaction between borate and sodium carbonate can take place at temperatures as low as about 600xc2x0 C. and can be carried out at temperatures up to at least 925xc2x0 C. Temperatures in the lower furnace of a recovery boiler where partial autocausticization would be expected to occur can range from as low as about 700 to 850xc2x0 C. in portions of the smelt to as high as 1100-1200xc2x0 C. in the gas phase or char bed. Thus a broad temperature range in which partial autocausticization may be carried out is between about 600xc2x0 and about 1200xc2x0 C. Typically the partial autocausticization reaction will occur automatically in the recovery boiler following the combustion of the organic-based black liquor. The combustion reaction leading to the formation of sodium carbonate is shown in equation (2) above. In order to achieve maximum reaction efficiency, the method of this invention requires that the molar ratio of boron (B) to carbonate (CO3) in the smelt, produced from combustion of the black liquor, be kept below 2:1, the stoichiometric requirement for full autocausticization according to equation (3), above. Preferably the boron to carbonate molar ratio is in the range of from about 0.01:1 to 1:1, more preferably in the range of from about 0.02:1 to 0.8:1, and most preferably in the range of from about 0.05:1 to 0.4:1. In addition, the molar ratio of sodium to boron should be above 3:1, preferably in the range of from about 4:1 to 400:1, more preferably in the range of from about 5:1 to 200:1 and most preferably in the range of from about 10:1 to 100:1. Under these conditions, the partial autocausticization reaction efficiency has been found to increase with increases in the sodium to boron ratio. However, very low boron to carbonate molar ratios, such as below about 0.01:1, and very high sodium to boron molar ratios, such as above about 400:1, would require very low levels of borate addition, such that minimal autocausticization of sodium carbonate would be achieved, despite the high reaction efficiency.
The borate requirements for partial autocausticization can be provided in a variety of inorganic borate forms including boric acid, boric oxide, and sodium borates such as sodium tetraborate and sodium metaborate and the various hydrated forms thereof. The preferred way of adding the borate into the process is to mix it into the spent (black) liquor or green liquor. It appears that an important factor in promoting the autocausticization reaction at high molar ratios of sodium to boron is the avoidance of excess sodium hydroxide in the reaction mixture prior to reaction, to avoid premature conversion of the borate reactants into autocausticization reaction products. However, the presence of such sodium hydroxide levels prior to recausticization would not be expected in normal wood pulping operations.
Another embodiment of the present invention is recausticization of a sodium carbonate-containing smelt by successive causticization steps including partial autocausticization of sodium carbonate, followed by a lime causticization step in which additional sodium carbonate is converted back to sodium hydroxide. Such an approach will avoid or minimize many of the problems associated with full autocausticizing such as high dissolved solids, high viscosity and low heating value of the black liquor by avoiding the need for high levels of borate in the recirculating liquor, while providing many of the benefits of autocausticizing. It will reduce the lime recovery circuit energy requirements or reduce the lime consumption in plants which do not recover lime. The process will therefore provide increased operating capacity in plants which are limited by the throughput capacity of an existing lime recovery circuit. The complete recausticization process begins with partial autocausticization of a sodium carbonate-containing smelt with borate, such as in a recovery boiler as described above, thereby producing a reacted portion and an unreacted portion of the smelt. Following partial autocausticization, the smelt is dissolved in water or aqueous liquor to make up an aqueous solution referred to as green liquor, thereby regenerating sodium hydroxide from the reacted portion of the smelt and dissolving the residual sodium carbonate from the unreacted portion. Lime, in the form of calcium oxide or calcium hydroxide, is added to the green liquor, containing the residual sodium carbonate. The lime converts dissolved sodium carbonate to sodium hydroxide and in the process the lime is converted to calcium carbonate. The resulting calcium carbonate mud can then be sent to a conventional lime recovery circuit for conversion back to calcium oxide if desired. The sodium hydroxide-containing solution, which is now referred to as white liquor, is ready for reuse in the delignification process or related pulping circuit.